二维数组和算法

1、⼆维数组

     1. 定义： int[][] scores = new int[][]{{80,90,100},{99,88,77},{66,100,88}};

     2. 简写：int[][] scores = {{80,90,100},{99,88,77},{66,100,88}};

     3. 定义⼆： int[][] scores = new int[3][3];

     4. 例⼦：

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String[][] name = new String[3][3];
        //赋值
        for (int i = 0; i < name.length; i++) {//i代表小组
            for (int j = 0; j < name[i].length; j++) {
                System.out.print("第" + (i + 1) + "组第" + (j + 1) + "个成员姓名为：");
                name[i][j] = scanner.next();
            }
        }
        for (int i = 0; i < name.length; i++) {//i代表小组
            for (int j = 0; j < name[i].length; j++) {
                System.out.println("第" + (i + 1) + "组第" + (j + 1) + "个成员姓名为：" + name[i][j]);
            }
        }
    }
}

2、 顺序查询 

      1. 需求：有⼀个数组，找⼀个数在不在这个数组中，如果在打印它的下标，如果不在，打印不存在。

      2.

import java.util.Scanner;

public class Demo02 {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5, 6};
        Scanner sc = new Scanner(System.in);
        System.out.print("请输入要查询的数：");
        int valuce = sc.nextInt();
        int index = -1;
        for (int i = 0; i < arr.length; i++) {
            if (valuce == arr[i]) {
                index = i;
            }
        }
        if (index == -1) {
            System.out.println("不存在");
        } else {
            System.out.println("存在，下标为：" + index);
        }
    }
}

3. 冒泡排序

    1. ⽐较经典的排序算法，⽐较两个相邻的元素，将较⼤的元素换到右边（后边，下边）。

    2.

//冒泡排序
public class Demo03 {
    public static void main(String[] args) {
        int[] arr = {99, 85, 79, 99, 69, 87};
        for (int i = 0; i < arr.length; i++) {
            for (int j = i + 1; j < arr.length; j++) {
                if (arr[i] > arr[j]) {
                    int temp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = temp;
                }
            }

        }
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
    }
}

4. ⼆分法

    1. 前提是有序序列

    2.

public class Demo04 {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        int value = 6;
        int index = -1;
        int right = arr.length - 1;
        int left = 0;
        int mid = (right + left) / 2;
        while (left <= right) {
            if (value == arr[mid]) {
                index = mid;
                break;
            } else if (value > arr[mid]) {
                left = mid + 1;
            } else if (value < arr[mid]) {
                right = mid - 1;
            }
            mid = (right + left) / 2;
        }
        if (index == -1) {
            System.out.println("不存在");
        } else {
            System.out.println("存在，下标为" + index);
        }
    }
}

5. arrays类

    1.

    2.

import java.util.Arrays;
public class Demo05 {
    public static void main(String[] args) {
        int[] arr1={10,20,30};
        int[] arr2={10,20,30};
        int[] arr3={10,40,30};
        System.out.println(Arrays.equals(arr1,arr2));//equals(数组1,数组2)比较两个数组是否相等
        System.out.println(Arrays.equals(arr1,arr3));
        System.out.println(Arrays.toString(arr1));//toString(数组)将一个数组转换为一个字符串
        int[] arr4=new int[3];
        Arrays.fill(arr4,100);//fill(数组,值)把数组中所有的元素都赋值为一个统一的值
        System.out.println(Arrays.toString(arr4));
        int[] arr5={10,20,30,40};
        int[] arr6=Arrays.copyOf(arr5,2);//copyOf(arr,length)把数组arr复制成一个长度为length的新数组
        System.out.println(Arrays.toString(arr6));
        Arrays.sort(arr5);
        System.out.println(Arrays.toString(arr5));
        System.out.println(Arrays.binarySearch(arr5,50));//binarySearch(arr,val)程序元素val的下标需要数组为有序的

    }
}

    3. 练习

import java.util.Scanner;

public class Demo06 {
    public static void main(String[] args) {
        int[][] scores = new int[3][5];
        int[] sum = new int[3];
        Scanner scanner = new Scanner(System.in);
        for (int i = 0; i < scores.length; i++) {
            System.out.println("***********第" + (i + 1) + "个班***********");
            for (int j = 0; j < scores[i].length; j++) {
                System.out.print("请输入第" + (j + 1) + "个学生的成绩：");
                scores[i][j] = scanner.nextInt();
                sum[i] += scores[i][j];
            }
        }
        System.out.println("***********成绩统计***********");
        for (int i = 0; i < sum.length; i++) {
            System.out.println((i + 1) + "班总成绩:" + sum[i]);
        }
    }
}